Instruments for the detection of certain combustible gases such as for example methane, ethylene, hydrogen and the like, normally employ a sensor which includes a pair of detector elements consisting of electrically conductive ceramic coated beads, one of which is a reference bead and the other of which is the measurement bead. The measurement bead is further coated with a catalytic composition, such as for example platinum, which, when contacted by a combustible gas in the presence of oxygen, catalyses an oxidation reaction resulting in the combustion of the gas. The beads are electrically connected in a conventional Wheatstone bridge and combustion of the gas at the measurement bead causes an increase in resistance through the measurement bead which throws the bridge out of equilibrium. The resulting output from the bridge is directly proportional to the concentration of the combustible gas and is read or recorded as an indication of the quantity of the combustible gas present in the sample flowing past the beads.
Conventionally, both the reference bead and the measurement bead are enclosed in shrouds or "cans" which are provided with apertures for communication between the interior of the can and the exterior. The gas sample is introduced through the aperture to the bead within the can. The purpose of the can is to protect the bead and to provide a relatively constant gas and thermal environment. However, the presence of the cans effectively isolates the reference bead from the measurement bead so that the beads may not be exposed to the same or similar compositional or thermal environments. Ambient conditions effect the resistance of the beads and differences in ambient conditions between the reference bead and the measurement bead will produce differences in their resistances due solely to the differing ambient conditions and not due to combustion of a sought for gas. Consequently, the resultant output from the sensor will produce erroneous results.
Although this problem has been recognized in the prior art and attempts have been made to provide a more uniform gas and thermal environment for the reference and the measurement beads. For example, a common practice is to provide a sleeve or cylinder which surrounds the shrouded reference and the measurement beads in an attempt to provide a more uniform gas mixture for contact with the beads within the cans. However, the use of a sleeve, although improving performance, has not proven entirely successful.